1. Field of the Invention
This invention relates to a picture transmission system for compressing picture information and for transmitting the compressed information, and more particularly to a picture transmission system for performing a scene change detection to detect changes in a scene on the transmitter side.
2. Description of the Related Art
When compressing picture information and transmitting the compressed information, predictive coding over two frames is effective for pictures of successive frames that are extremely similar to each other. For example, inter-frame difference coding which encodes a difference between a current frame and the former frame is effective. On the other hand, when motion (movement) within a picture is great, intra-frame predictive coding (predictive coding within the same frame) is more suitable. In accordance with the coding system of the H.261 which is the picture coding standard of television conference and visual telephone of CCITT (Comite Consultatif International Telegraphique et Telephonique), there is employed a scheme in which plural coding systems are combined to suitably carry out coding, the coding systems being switched depending upon the state of picture.
When scenes change or different pictures are inserted, a scene change phenomenon may occur during the transmission of a picture, particularly where no correlation exists between a certain frame and the former frame. When scene change phenomenon is experienced, the coding mode automatically becomes INTRA-MODE (intra-frame correlation mode) in accordance with the coding system of the H.261 standard. Thus, the transmission state is temporarily switched to the coding transmission state of the entire picture using a quantized value at that time. As a result, the amount of data to be transmitted is remarkably increased and the transmission time is prolonged. For this reason, there was the problem that a portion of picture may be placed in freeze state, or the frequency of frame dropping (missing frames) may increase.